LPG Fractionation Recovery System

ABSTRACT

A system for recovering an LPG fraction according to the present invention includes: a compressor increasing the pressure of excess gas such as associated gas; a deethanizer column dissolving, in a sponge oil, propane and butane contained in the excess gas and separating a gas having a carbon number of two or less from the excess gas; a sponge oil transfer pipe discharging the sponge oil from the deethanizer column; a debutanizer column separating propane and butane from the sponge oil; an LPG fraction discharge pipe discharging the fraction containing propane and butane from the debutanizer column; and a sponge oil return pipe returning the sponge oil from the debutanizer column to the deethanizer column.

TECHNICAL FIELD

The present invention relates to a system for recovering an LPG fraction, and more particularly to a system for effectively using excess gas generated during the production of crude oil in an oil field.

BACKGROUND ART

In a plant for producing crude oil from oil reservoirs, excess gas referred to as associated gas is generated. The associated gas is generally a gas containing hydrocarbons having a carbon number of 1 to 5, and is generated when the pressure of the hydrocarbons dissolved in crude oil under high pressure in oil reservoirs is reduced in a separator. The associated gas has a feature that it contains, as main components, methane and ethane, and that it has a low content of hydrocarbons of propane, and the like, which have a carbon number of 3 or more and which are effective as LPG (Liquefied petroleum gas). Conventionally, the associated gas is processed by combustion (flare), or is again press-fed into the oil reservoirs.

Under the background of increasing concern about global warming and increasing gas cost, various attempts have been made to effectively use the associated gas. For example, Patent Literature 1 describes a method in which a product gas containing carbon dioxide, methane, and the like, and NGL (Natural Gas Liquid) containing, as main components, ethane, propane, and heavy components are produced from natural gas or associated gas from crude oil by using a membrane separation method, an absorption separation method, and a distillation separation method.

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Patent Laid-Open Publication No.     2005-290151

SUMMARY OF INVENTION Technical Problem

Meanwhile, the associated gas and the natural gas have a problem that they are difficult to be transported as compared with crude oil. In a large oil and gas field, it is possible to construct a pipeline and an LNG plant for transporting the natural gas produced in the oil and gas field. However, in a region where middle and small scaled oil fields are dotted, or in an offshore oil field, it is not profitable to construct a pipeline or an LNG plant for the associated gas, and hence, in the present situation, the effective use of the associated gas is not sufficiently achieved.

The present invention has been made in view of the above described circumstances. An object of the present invention is to provide a system for recovering an LPG fraction, which system can efficiently recover the LPG fraction from the associated gas, and which system is useful for the effective use of the associated gas in an oil field of such the scale that the conventional liquefaction process is not profitably used.

Solution to Problem

A system for recovering an LPG fraction according to the present invention comprises: a compressor increasing the pressure of all or a part of excess gas generated during the production of crude oil, the excess gas containing propane and butane; a deethanizer column bringing the pressurized excess gas into gas-liquid contact with a sponge oil so that propane and butane contained in the excess gas can be dissolved in the sponge oil, and a gas having a carbon number of two or less can be separated from the excess gas; a sponge oil transfer pipe discharging the sponge oil with propane and butane dissolved therein from the deethanizer column; a debutanizer column connected to the deethanizer column via the sponge oil transfer pipe and separating propane and butane from the sponge oil; an LPG fraction discharge pipe discharging the fraction containing propane and butane from the debutanizer column; and a sponge oil return pipe discharging the sponge oil from the debutanizer column and returning the sponge oil to the deethanizer column.

With the above-described system for recovering the LPG fraction, the associated gas (excess gas) is subjected to the processes in the deethanizer column and the debutanizer column, and thereby it is possible to sufficiently reduce the content of the gas which has a carbon number of two or less (methane and ethane) and then the fluid containing the LPG fraction is discharged from the debutanizer column. When the content of these components is sufficiently reduced, the liquefied LPG fraction can be transported by an ordinary tank truck, or the like, and hence the associated gas can be effectively used even in an oil field of such a scale that the conventional liquefaction process cannot be profitably used. When the fluid having a carbon number of two or less is not sufficiently reduced, a special tank truck, or the like, having a high pressure resistance needs to be prepared for the transportation of the LPG fraction. Note that LNG containing methane as a main component is liquefied by being cooled to about −160° C., while the LPG fraction is comparatively easily liquefied by being pressurized.

Here, the “LPG fraction” means a fraction in which propane (C3) and butane (C4) are contained as main components and in which the content of methane (C1) and ethane (C2) is sufficiently low. For example, a fraction, in which propane and butane are contained as main components, and in which the total amount of methane and ethane is about 5 mol % or less and the total amount of hydrocarbons having a carbon number of five or more (hereinafter referred to as “heavy fraction”) is about 30 mol % or less, is an LPG fraction. When the excess gas contains a heavy fraction, the heavy fraction is dissolved in the sponge oil in the deethanizer column together with propane and butane, and is then separated from the sponge oil in the debutanizer column.

The deethanizer column and the debutanizer column can be operated if heating methods and circulation methods of the sponge oil are provided. Thus, from viewpoints of operating costs and construction costs, the deethanizer column and the debutanizer column have an advantage that they can be installed in a plant in an oil field more easily as compared with the cryogenic separation system requiring advanced cooling techniques. Here, the sponge oil means the oil which dissolves therein hydrocarbons having a carbon number of three or more in the deethanizer column. Depending on a chemical composition of the excess gas, it is possible to use, as the sponge oil, for example, jet fuel or kerosene, a fraction corresponding to jet fuel or kerosene, or a mixture of hydrocarbons having a carbon number of 7 to 16, or normal paraffin or iso-paraffin having a carbon number of 10 to 14.

In the present invention, from a viewpoint of thermal efficiency, it is preferred to use the excess gas as fuel. That is, it is preferred that the compressor is driven by a motor or gas engine using the excess gas as fuel. Further, it is preferred that the above-described system for recovering the LPG fraction further comprises a first circulation pipe branched from the sponge oil transfer pipe and returning a part of the sponge oil from the deethanizer column to the deethanizer column; and a first reboiler arranged in the middle of the first circulation pipe; and that the first reboiler has a first main body section receiving the sponge oil; and a first heating pipe provided so as to pass through the inside of the first main body section and allowing a fluid heated by combustion of the excess gas to flow therethrough. It is preferred that the above-described system for recovering the LPG fraction further comprises a second circulation pipe branched from the sponge oil return pipe and returning a part of the sponge oil from the debutanizer column to the debutanizer column; and a second reboiler arranged in the middle of the second circulation pipe; and that the second reboiler has a second main body section receiving the sponge oil; and a second heating pipe provided so as to pass through the inside of the second main body section and allowing a fluid heated by combustion of the excess gas to flow therethrough.

The above-described system for recovering the LPG fraction may further comprise a depentanizer column supplied with the fraction having a carbon number of three or more from the debutanizer column through the LPG fraction discharge pipe, and separating a fraction having a carbon number of five or more from the supplied fraction. By the use of the system having such configuration, an LPG fraction consisting substantially of propane and butane can be obtained, so that commercial grade of LPG can be available.

Advantageous Effects of Invention

With the present invention, it is possible to provide a system for recovering an LPG fraction, which system can efficiently recover the LPG fraction from the associated gas and which system is useful for the effective use of the associated gas produced in an oil field of such a scale that the conventional liquefaction process cannot be profitably used.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is the schematical flow diagram that shows a preferred embodiment of a system for recovering an LPG fraction according to the present invention;

FIG. 2 is the cross-sectional view that shows schematical configuration of a reboiler provided in the subsequent stage of the deethanizer column; and

FIG. 3 is the schematical view that shows another embodiment of the system for recovering the LPG fraction according to the present invention.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments according to the present invention will be described in detail with reference to the attached drawings.

A system 50 shown in FIG. 1 is for recovering an LPG fraction from the excess gas such as the associated gas which is generated during the production of crude oil in an oil field. The system 50 for recovering the LPG fraction is configured to process, as an object to be processed, all or a part of the excess gas, and is configured to be supplied with the gas to be processed, through a pipe L2 branched from a pipe L1 for transferring the excess gas to a flare stack (not shown).

The chemical composition of associated gas is different in each oil field, but the associated gas having the following composition ranges can be used as the gas to be processed:

methane (C1): 30 to 85 mol %,

ethane (C2): 10 to 50 mol %,

propane (C3): 1 to 40 mol %,

butane (C4): 1 to 40 mol %,

pentane (C5): 1 to 20 mol %,

hydrocarbons (C6+) having a carbon number of six or more: 0 to 10 mol %.

The system 50 for recovering the LPG fraction mainly includes: two-stage compressors 11 and 12 which increase the pressure of the feed gas; a deethanizer column 20 which separates components having a carbon number of two or less from the gas by using a sponge oil; a sponge oil transfer pipe L5 which discharges the sponge oil from the deethanizer column 20; a debutanizer column 30 which is installed in the subsequent stage of the deethanizer column 20; an LPG fraction discharge pipe L6 which discharges the LPG fraction (fraction containing propane and butane) from the debutanizer column 30; and a sponge oil return pipe L7 which returns the sponge oil to the deethanizer column 20 from the debutanizer column 30.

The compressors 11 and 12 are configured such that the pressure of the gas to be processed, which is supplied at a low pressure (for example, 100 to 250 kPa) through the pipe L2, is increased to about 500 to 2000 kPa. It is preferred that the compressors 11 and 12 are driven by a gas engine GE using the excess gas as fuel. Note that cooling systems 11 a and 12 a and separators 13, 14 and 15 for separating water and condensate from the gas to be processed are respectively installed at the preceding and subsequent stages of the compressors 11 and 12. Preferred cooling system includes an air-cooling type cooling system and a water-cooling type cooling system, and an air fin cooler and a water circulation type cooler are preferred in particular in a region sufficient water is not available.

The deethanizer column 20 is configured to separate methane and ethane (gas having a carbon number of two or less) from the gas to be processed in such a manner that the gas to be processed, which is pressurized by the compressors 11 and 12, is brought into gas-liquid contact with the sponge oil so that propane, butane, and the heavy fraction, which are contained in the gas to be processed, can be absorbed in the sponge oil. The sponge oil is supplied from the sponge oil return pipe L7 connected to an upper portion of the deethanizer column 20. The methane and ethane, which are not absorbed by the sponge oil, are discharged from a pipe L4 which is connected on a top portion of the deethanizer column 20 to the flare stack. On the other hand, the sponge oil, in which propane, butane, and the heavy fraction are absorbed, is discharged to the sponge oil transfer pipe L5 connected on a bottom portion of the deethanizer column 20. Note that the reboiler is provided at a bottom portion of the deethanizer column 20, and C1 and C2 absorbed in the sponge oil are efficiently removed from the sponge oil.

In the case where jet fuel or kerosene, or a fraction corresponding to jet fuel or kerosene, or a mixture of hydrocarbons having a carbon number of 7 to 16, or normal paraffin or isoparaffin having a carbon number of 10 to 14 is used as the sponge oil, it is preferred that the temperature in the deethanizer column 20 is set to 100 to 220° C., and that the pressure in the deethanizer column 20 is set to 700 to 2000 kPa.

As shown in FIG. 1, a circulation pipe (first circulation pipe) L5 a is branched from the middle of the sponge oil transfer pipe L5 so that a part of the sponge oil can be returned to the deethanizer column 20 through the circulation pipe L5 a. A reboiler (first reboiler) 16 is arranged in the middle of the circulation pipe L5 a. The amount of methane and ethane, which flow into the debutanizer column 30 together with the sponge oil, can be more surely reduced by the circulation pipe L5 a and the reboiler 16.

As shown in FIG. 2, the reboiler 16 includes a main body section (first main body section) 16 a which stores the sponge oil, and a heating pipe (first heating pipe) 16 b which is provided to pass through the inside of the main body section 16 a and to allow an oil heated by combustion of the excess gas to flow therethrough. The sponge oil stored in the main body section 16 a is heated by contact with the outer surface of the heating pipe 16 b and is returned to the deethanizer column 20 in the state of a gas-liquid mixed phase. Note that the combustion gas of the excess gas may also be made to directly flow through the heating pipe 16 b instead of the hot oil.

The distal end of the sponge oil transfer pipe L5 is connected to the middle stage of the debutanizer column 30 in which the LPG fraction is separated from the sponge oil supplied through the pipe L5. The LPG fraction evaporated from the sponge oil in the debutanizer column 30 is discharged from the LPG fraction discharge pipe L6 connected on a top portion of the debutanizer column 30. On the other hand, the sponge oil, from which the LPG fraction is separated, is discharged from the sponge oil return pipe L7 connected on a bottom portion of the debutanizer column 30.

In the case where jet fuel or kerosene, or a fraction corresponding to jet fuel or kerosene, or a mixture of hydrocarbons having a carbon number of 7 to 16, or normal paraffin or isoparaffin having a carbon number of 10 to 14 is used as the sponge oil, it is preferred that the temperature in the debutanizer column 30 is set to 150 to 300° C., and that the pressure in the debutanizer column 30 is set to 500 to 1500 kPa.

As shown in FIG. 1, a circulation pipe (second circulation pipe) L7 a is branched from the middle of the sponge oil return pipe L7 so that a part of the sponge oil can be returned to the debutanizer column 30 through the circulation pipe L7 a. A reboiler (second reboiler) 26 is arranged in the middle of the circulation pipe L7 a. By the circulation pipe L7 a and the reboiler 26, the amount of the LPG fraction returned to the deethanizer column 20 together with the sponge oil is reduced so that the yield of the LPG fraction can be improved. Note that a heat exchanger 22 and a cooling system 23 are provided in the middle of the sponge oil return pipe L7.

The reboiler 26 has the same configuration as that of the reboiler 16 shown in FIG. 2. The reboiler 26 includes a main body section (second main body section) 26 a which stores the sponge oil, and a heating pipe (second heating pipe) 26 b which is provided to pass through the inside of the main body section 26 a and to allow an oil heated by combustion of the excess gas to flow therethrough. The sponge oil in the main body section 26 a is heated by contact with the outer surface of the heating pipe 26 b and is returned to the debutanizer column 30 in the state of a gas-liquid mixed phase. Note that the combustion gas of the excess gas may also be made to directly flow through the heating pipe 26 b instead of the hot oil.

The LPG fraction discharge pipe L6 is configured to discharge the LPG fraction from the debutanizer column 30, so as to transfer the LPG fraction to a separator 32. A condenser 31 is provided in the middle of the LPG fraction discharge pipe L6. All or a part of the LPG fraction condensed by the cooling is temporarily stored in the separator 32. The LPG fraction subjected to the processing in the separator 32 is transported to a LPG tank 45 through a pipe L9. When the amount of the LPG fraction stored in the LPG tank 45 reaches a fixed amount, the LPG fraction is transferred to a tank truck, so as to be transported to a refinery plant in which the LPG fraction is further refined into commercial products. In the LPG tank 45, the LPG fraction can be stored under the conditions of a temperature of 30 to 60° C. and a pressure of 500 to 1800 kPa. Note that the purity of the LPG fraction can be increased by refluxing a part of the LPG fraction discharged from the separator 32 to the debutanizer column 30 through a pipe L9 a.

With the system 50 for recovering the LPG fraction, it is possible to efficiently remove methane and ethane from the associated gas, and to stably obtain the LPG fraction which has a concentration exceeding 99 volume % (and which includes propane, butane, and a fraction having a carbon number of five or more when the associated gas contain the fraction). The LPG fraction having such chemical composition can be transported by an ordinary tank truck. Therefore, even in an area where small and middle scale oil fields exist, LPG products can be commercially produced in such a manner that the system 50 for recovering the LPG fraction is installed in each of the oil fields or gathering station, and that the LPG fraction is collected in a refinery plant from each of the LPG tanks 45. In the refinery plant, a process, such as a process to remove a fraction having a carbon number of five or more remaining in the LPG fraction, is performed. Note that, in some cases, the LPG fraction may also be transported by a pipeline, a tanker, or the like, instead of the tank truck.

The system 50 for recovering the LPG fraction is featured in that the LPG fraction contained in the associated gas can be recovered at the recovery rate of 70 to 99%, and in that the energy supply from the outside can be eliminated by using the flare gas (excess gas) as an energy source. Further, the system 50 for recovering the LPG fraction is also featured in that the separation of C1 and C2 hydrocarbons from C3 to C6 hydrocarbons can be achieved without generating the freezing point temperature or the cryogenic temperature by a refrigerator, self-refrigeration, or the like.

The system 50 for recovering the LPG fraction can also be applied to an offshore oil field. In this case, it is only necessary to mix the LPG fraction recovered from the associated gas with a crude oil line or a condensate line (fraction of C4, C5, and the like) of the oil production facility. Thereby, the LPG fraction can be used as stock oil for products. In this case, there is an advantage that an LPG ship requiring high transportation costs need not be used in order to transport the recovered LPG fraction.

In the above, the preferred embodiment according to the present invention is described, but the present invention is not limited to the above described embodiment. For example, in the above described embodiment, the case is exemplified in which the LPG fraction is transported to a refinery plant so as to be further processed in the refinery plant. Such processing may also be performed in the system for recovering the LPG fraction. An system 60 for recovering an LPG fraction shown in FIG. 3 includes, in the subsequent stage of the debutanizer column 30, a depentanizer column 40 for separating a fraction (C5+) having a carbon number of five or more from the LPG fraction. The system 60 for recovering the LPG fraction has the same configuration as that of the system 50 for recovering the LPG fraction except for being provided with the depentanizer column 40. With the system 60 for recovering the LPG fraction, the LPG fraction consisting substantially of propane and butane can be obtained, so that commercial grade of LPG can be available.

In the above described embodiment, the case is exemplified, in which the compressors are provided in two stages in order to increase the pressure of the gas to be processed. However, the number of stages of compressors is not limited to two, and for example, a compressor may also be provided in single stage. Further, the compressor is not limited to the type which is driven by the gas engine GE, and, for example, a compressor driven by a motor may also be adopted. As the motor, it is preferred to use an engine generator using the excess gas as fuel.

INDUSTRIAL APPLICABILITY

According to the present invention, an system for recovering an LPG fraction is provided, which system can efficiently recover the LPG fraction from the associated gas, and which system is useful for the effective use of the associated gas in an oil field of such the scale that the conventional liquefaction process is not profitably used.

REFERENCE SINGS LIST

-   11, 12: compressor; 16: reboiler (first reboiler); 16 a: main body     section (first main body section); 16 b: heating pipe (first heating     pipe); 20: deethanizer column; 26: reboiler (second reboiler); 26 a:     main body section (second main body section); 26 b: heating pipe     (second heating pipe); 30: debutanizer column; 40: depentanizer     column; 45: LPG tank; 50, 60: system for recovering an LPG fraction;     GE: gas engine; L5: sponge oil transfer pipe; L5 a: circulation pipe     (first circulation pipe); L6: LPG fraction discharge pipe; L7:     sponge oil return pipe; L7 a: circulation pipe (second circulation     pipe). 

1. A system for recovering an LPG fraction comprising: a compressor increasing the pressure of all or a part of excess gas generated during the production of crude oil, the excess gas containing propane and butane; a deethanizer column bringing the pressurized excess gas into gas-liquid contact with a sponge oil so that propane and butane contained in the excess gas can be dissolved in the sponge oil, and a gas having a carbon number of two or less can be separated from the excess gas; a sponge oil transfer pipe discharging from the deethanizer column, the sponge oil with propane and butane dissolved therein; a debutanizer column connected to the deethanizer column via the sponge oil transfer pipe, and separating propane and butane from the sponge oil; an LPG fraction discharge pipe discharging the fraction containing propane and butane from the debutanizer column; and a sponge oil return pipe discharging the sponge oil from the debutanizer column and returning the sponge oil to the deethanizer column.
 2. The system for recovering the LPG fraction according to claim 1, wherein the compressor is driven by a motor or a gas engine using the excess gas as fuel.
 3. The system for recovering the LPG fraction according to claim 1, further comprising: a first circulation pipe branched from the sponge oil transfer pipe and returning a part of the sponge oil from the deethanizer column to the deethanizer column; and a first reboiler arranged in the middle of the first circulation pipe, wherein the first reboiler has a first main body section receiving the sponge oil; and a first heating pipe provided so as to pass through the inside of the first main body section and allowing a fluid heated by combustion of the excess gas to flow through the first heating pipe.
 4. The system for recovering the LPG fraction according to claim 1, further comprising: a second circulation pipe branched from the sponge oil return pipe and returning a part of the sponge oil from the debutanizer column to the debutanizer column; and a second reboiler arranged m the middle of the second circulation pipe, wherein the second reboiler has a second main body section receiving the sponge oil, and a second heating pipe provided so as to pass through the inside of the second main body section and allowing a fluid heated by combustion of the excess gas to flow through the second heating pipe.
 5. The system for recovering the LPG fraction according to claim 1, further comprising a depentanizer column supplied with a fraction having a carbon number of three or more from the debutanizer column through the LPG fraction discharge pipe, and separating a fraction having a carbon number of five or more from the supplied fraction.
 6. The system for recovering the LPG fraction according to claim 1, wherein the sponge oil is jet fuel or kerosene, or fraction corresponding to jet fuel or kerosene, or a mixture of hydrocarbons having a carbon number of 7 to 16, or normal paraffin or iso-paraffin having a carbon number of 10 to
 14. 7. The system for recovering the LPG fraction according to one claim 1, wherein the recovery rate of LPG fraction contained in the excess gas is 70 to 99%.
 8. The system for recovering the LPG fraction according to claim 2, further comprising: a first circulation pipe branched from the sponge oil transfer pipe and returning a part of the sponge oil from the deethanizer column to the deethanizer column; and a first reboiler arranged in the middle of the first circulation pipe, wherein the first reboiler has a first main body section receiving the sponge oil; and a first heating pipe provided so as to pass through the inside of the first main body section and allowing a fluid heated by combustion of the excess gas to flow through the first heating pipe.
 9. The system for recovering the LPG fraction according to claim 2, further comprising: a second circulation pipe branched from the sponge oil return pipe and returning a part of the sponge oil from the debutanizer column to the debutanizer column; and a second reboiler arranged m the middle of the second circulation pipe, wherein the second reboiler has a second main body section receiving the sponge oil, and a second heating pipe provided so as to pass through the inside of the second main body section and allowing a fluid heated by combustion of the excess gas to flow through the second heating pipe.
 10. The system for recovering the LPG fraction according to claim 2, further comprising a depentanizer column supplied with a fraction having a carbon number of three or more from the debutanizer column through the LPG fraction discharge pipe, and separating a fraction having a carbon number of five or more from the supplied fraction.
 11. The system for recovering the LPG fraction according to claim 2, wherein the sponge oil is jet fuel or kerosene, or fraction corresponding to jet fuel or kerosene, or a mixture of hydrocarbons having a carbon number of 7 to 16, or normal paraffin or iso-paraffin having a carbon number of 10 to
 14. 12. The system for recovering the LPG fraction according to claim 2, wherein the recovery rate of LPG fraction contained in the excess gas is 70 to 99%.
 13. The system for recovering the LPG fraction according to claim 3, further comprising: a second circulation pipe branched from the sponge oil return pipe and returning a part of the sponge oil from the debutanizer column to the debutanizer column; and a second reboiler arranged m the middle of the second circulation pipe, wherein the second reboiler has a second main body section receiving the sponge oil, and a second heating pipe provided so as to pass through the inside of the second main body section and allowing a fluid heated by combustion of the excess gas to flow through the second heating pipe.
 14. The system for recovering the LPG fraction according to claim 3, further comprising a depentanizer column supplied with a fraction having a carbon number of three or more from the debutanizer column through the LPG fraction discharge pipe, and separating a fraction having a carbon number of five or more from the supplied fraction.
 15. The system for recovering the LPG fraction according to claim 3, wherein the sponge oil is jet fuel or kerosene, or fraction corresponding to jet fuel or kerosene, or a mixture of hydrocarbons having a carbon number of 7 to 16, or normal paraffin or iso-paraffin having a carbon number of 10 to
 14. 16. The system for recovering the LPG fraction according to claim 3, wherein the recovery rate of LPG fraction contained in the excess gas is 70 to 99%.
 17. The system for recovering the LPG fraction according to claim 4, further comprising a depentanizer column supplied with a fraction having a carbon number of three or more from the debutanizer column through the LPG fraction discharge pipe, and separating a fraction having a carbon number of five or more from the supplied fraction.
 18. The system for recovering the LPG fraction according to claim 4, wherein the sponge oil is jet fuel or kerosene, or fraction corresponding to jet fuel or kerosene, or a mixture of hydrocarbons having a carbon number of 7 to 16, or normal paraffin or iso-paraffin having a carbon number of 10 to
 14. 19. The system for recovering the LPG fraction according to claim 4, wherein the recovery rate of LPG fraction contained in the excess gas is 70 to 99%.
 20. The system for recovering the LPG fraction according to claim 5, wherein the sponge oil is jet fuel or kerosene, or fraction corresponding to jet fuel or kerosene, or a mixture of hydrocarbons having a carbon number of 7 to 16, or normal paraffin or iso-paraffin having a carbon number of 10 to
 14. 